Cardiac event detection over varying time scale

ABSTRACT

Disclosed is a “tracker system” that includes implanted electrical leads which are part of an implanted cardiotracker plus external equipment that includes external alarm means and a physician&#39;s programmer. The tracker system is designed to monitor the degradation of a patient&#39;s cardiovascular condition from one or more causes. These causes include the rejection of a transplanted heart and/or the progression of a stenosis in a coronary artery. As one or more stenoses in a coronary artery become progressively more narrow thereby causing reduced blood flow to the heart muscle coronary circulation, the tracker system can alert the patient by either or both internal and/or external alarm means to take the appropriate medical action. The physician&#39;s programmer can be used to display histograms of key heart signal parameters that are indicative of the patient&#39;s cardiovascular condition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of patent application Ser. No.10/950,401 filed on 28 Sep. 2004 now U.S. Pat. No. 7,512,438 which is apatent application based upon Provisional Patent Application Ser. No.60/524,873, entitled “System for Monitoring Cardiovascular Condition”filed provisionally on 26 Nov. 2003.

FIELD OF USE

This invention is in the field of systems that monitor a patient'scardiovascular condition using implanted devices that interact withother devices located externally to the patient.

BACKGROUND OF THE INVENTION

Heart disease is the leading cause of death in the United States. Aheart attack (also known as an acute myocardial infarction (AMI))typically results from a thrombus (i.e., a blood clot) that obstructsblood flow in one or more coronary arteries. AMI is a common andlife-threatening complication of coronary artery disease. Myocardialischemia is caused by an insufficiency of oxygen to the heart muscle.Ischemia is typically provoked by physical activity or other causes ofincreased heart rate when at least one coronary artery is narrowed byatherosclerosis. Patients will often (but not always) experience chestdiscomfort (angina) when the heart muscle is experiencing ischemia.Those with coronary atherosclerosis are at higher risk for AMI if theplaque becomes further obstructed by thrombus. Those patients who do nothave any symptom of ischemia or AMI are said to have “silent ischemia.”These patients are at the highest risk of dying from coronary arterydisease.

The current treatment for a coronary artery narrowing (a stenosis) isthe insertion of a drug eluting stent such as the Cypher™sirolimus-eluting stent from Cordis Corporation or the Taxus™paclitaxel-eluting stent from the Boston Scientific Corporation. Theinsertion of a stent into a stenosed coronary artery is the mostreliable medical treatment to eliminate or reduce coronary ischemia andto prevent the complete blockage of a coronary artery, which completeblockage results in an AMI.

Acute myocardial infarction and ischemia may be detected from apatient's electrocardiogram (ECG) by noting an ST segment shift (i.e.,voltage change) over a relatively short (less than 5 minutes) period oftime after a complete blockage of a coronary artery. However, withoutknowing the patient's normal (i.e., baseline) ECG pattern, detectionfrom a standard 12 lead ECG can be unreliable.

Fischell, et al in U.S. Pat. Nos. 6,112,116, 6,272,379 and 6,609,023describe implantable systems and algorithms for detecting the onset ofacute myocardial infarction and providing both treatment and patientalerting. While Fischell, et al discuss the acute detection of a shiftin the ST segment of the patient's electrogram from an electrode withinthe heart as the trigger for alarms, it may be desirable to provide moresophisticated long term tracking of myocardial ischemia to provide earlyprediction of coronary obstruction before the occurrence of a completecoronary artery blockage that results in an AMI. An important aspect ofthe Fischell, et al patents is that the heart's electrical signal frominside the patient's body, which is called an “electrogram,” is a moreaccurate means to discern ischemia as compared to the heart's signal asmeasured on the patient's skin which is the ECG.

The Fischell, et al patents as listed above discuss the storage ofrecorded electrograms and/or electrocardiogram data; however techniquesto optimally capture the appropriate statistical electrogram and/orelectrocardiogram data over days, weeks and months in a limited amountof system memory are not described.

The Reveal™ subcutaneous loop Holter monitor sold by Medtronic, Inc.uses two case electrodes spaced about 3 inches apart to recordelectrogram information. Recording can be triggered automatically whenarrhythmias are detected or upon patient initiation using an externaldevice. The Reveal is designed to record electrogram data and does notinclude the signal processing capability to track changes in the heartsignal over an extended period of time. The Reveal also does not havethe capability to measure ST segment shift. In fact, its high passfiltering and electrode spacing preclude accurate detection of changesin the low frequency aspects of the heart's electrical signal, which lowfrequency aspects are required for the detection of ischemia.

While pacemakers track the numbers of beats paced or not paced andpacemaker programmers can display the beat data in histogram format,pacemakers do not produce histograms of heart signal parameters relatedto the electrogram wave form. In other words, pacemakers track pacemakeroperation but pacemakers do not measure or compute heart signalparameters of the beats in the electrogram signal, nor do they save thecomputed values of heart signal parameters in memory.

Pacemakers have been used to collect intramyocardial electrogram (IMEG)data for the purpose of using a decrease in electrogram QRS complexvoltage as an indicator of the rejection of a transplanted heart. Theexpense, patient discomfort and inconvenience of endomyocardial biopsyto detect heart transplant rejection makes an electronic method highlydesirable. The published paper “Clinical Heart Transplantation withoutRoutine Endomyocardial Biopsy” by Warnecke, et al in theNovember/December 1992 issue of The Journal of Heart and LungTransplantation showed that IMEG recordings made with a cardiacpacemaker have the potential to replace endomyocardial biopsy (EMB) as adiagnostic method to detect transplant rejection. Specifically, Warneckeet al showed that an 8% decline in IMEG voltage provided the bestsensitivity and specificity as an indicator of potential acute moderateallograft rejection of a transplanted heart. Unfortunately, pacemakersare not designed to collect weeks or months of statistical data onelectrogram voltage variations. The additional external supportequipment needed to continually offload the raw electrogram data from apacemaker is expensive and inconvenient to use.

The term “medical practitioner” shall be used herein to mean any personwho might be involved in the medical treatment of a patient. Such amedical practitioner would include, but is not limited to, a medicaldoctor (e.g., a general practice physician, an internist or acardiologist), a medical technician, a paramedic, a nurse or anelectrogram analyst. Although the masculine pronouns “he” and “his” areused herein, it should be understood that the patient or medicalpractitioner could be a man or a woman. A “cardiac event” includes anacute myocardial infarction, ischemia caused by effort (such asexercise) and/or an elevated heart rate, bradycardia, tachycardia or anarrhythmia such as atrial fibrillation, atrial flutter, ventricularfibrillation, premature ventricular contractions or premature atrialcontractions (PVCs or PACs) and the rejection of a transplanted heart.

For the purpose of this invention, the term “electrocardiogram” isdefined to be the heart's electrical signal as sensed through skinsurface electrodes that are placed in a position to indicate the heart'selectrical activity (depolarization and repolarization). Anelectrocardiogram segment refers to electrocardiogram data for either aspecific length of time, such as 10 seconds, or a specific number ofheart beats, such as 10 beats. For the purposes of this specification,the PQ segment of a patient's electrocardiogram is the typically flatsegment of a beat of an electrocardiogram that occurs just before the Qand R waves. For the purposes of this specification the ST segment of apatient's electrocardiogram is that segment of a beat of anelectrocardiogram that occurs just after the S wave.

Although occasionally described as an electrocardiogram (ECG), theelectrical signal from the heart as measured from electrodes within thebody is properly termed an “electrogram” or intramyocardial electrogram(IMEG). For the purpose of this invention, the term “electrogram” isdefined to be the heart's electrical signal from one or more implantedelectrode(s) that are placed in a position to indicate the heart'selectrical activity (depolarization and repolarization). An “electrogramsegment” refers to a recording of electrogram data for either a specificlength of time, such as 10 seconds, or a specific number of heart beats,such as 10 beats. For the purposes of this specification the PQ segmentof a patient's electrogram is the typically flat, generally horizontalsegment of an electrogram that occurs just before the Q and R waves. Forthe purposes of this specification the ST segment of a patient'selectrogram is that segment of an electrogram that occurs just after theS wave. For the purposes of this specification, the term QRS voltage isdefined as a measure of QRS complex voltage amplitude which may eitherbe measured from Q to R, or S to R of a beat of the electrogram. For thepurposes of this specification, the term QRS segment or QRS complex isthat segment of the electrogram from the Q through the R and ending atthe J point of the S wave. For the purposes of this specification, theterms “detection” and “identification” of a cardiac event have the samemeaning. A beat is defined as a sub-segment of an electrogram orelectrocardiogram segment which covers the electrical signal from theheart for exactly one beat of the heart and includes exactly one R wave.If the heart rate is 60 bpm, then the sub-segment of the electrogramthat is exactly one beat would represent a sub-segment of theelectrogram that is exactly 1.0 second in duration. For the purposes ofthis invention, the term “average value”, “average amplitude” or“average voltage” of any segment (viz., QRS complex, ST segment or PQsegment) of the electrogram shall be defined as meaning either the meanor the median of a multiplicity of measurements of that segment. It isalso envisioned that in some cases both mean and median may be computedand will on occasion be described separately herein.

“Heart signal parameters” are defined to be any measured or calculatedvalue created during the processing of one or more beats of electrogram(or electrocardiogram) data. Heart signal parameters are features of theelectrogram derived from one or more measured values and include PQsegment average voltage, ST segment average voltage, R wave peakvoltage, ST deviation (ST segment average voltage minus PQ segmentaverage voltage), ST shift (ST deviation compared to a baseline averageST deviation taken at some prior time), average signal strength, T wavepeak height, T wave average voltage, T wave deviation, QRS complexwidth, QRS voltage, heart rate and R-R interval. Counts of the number ofarrhythmia related events such as PACs, PVCs and/or episodes of atrialfibrillation are not considered herein to be heart signal parameters asthey do not directly result from a measured value derived from a beat ofthe electrogram. ST segment related heart signal parameters include, STsegment average voltage, ST deviation and ST shift.

SUMMARY OF THE INVENTION

A “tracker system” as defined herein includes implanted electrical leadswhich are part of an implanted cardiotracker plus external equipmentthat includes external alarm means and a physician's programmer. Thepresent invention is the tracker system for monitoring the degradationof a patient's cardiovascular condition from one or more causes. Thesecauses include the rejection of a transplanted heart and further includethe progression of a stenosis in a coronary artery; e.g., as one or morestenoses in a coronary artery become progressively more narrow therebycausing reduced blood flow to the heart muscle. As less and less bloodis available to the heart muscle, the patient's ST segment will shiftduring exertion by an ever increasing amount. Eventually, if thestenosis severely restricts blood flow or a plaque rupture occurs, athrombus can form causing an AMI. By noting changes over time in theshift of ST segment voltage in relation to the patient's heart rate, thepatient's doctor can identify coronary artery narrowing and intervenebefore a potentially fatal AMI occurs. The preferred intervention forsuch narrowing is the implantation of one or more drug eluting stents torestore normal blood flow for the coronary circulation. The trackersystem also has the capability for tracking electrogram signal amplitude(e.g., QRS voltage) as well as electrogram feature time durations suchas the width of the QRS complex, etc. A decrease in the average value ofthe QRS voltage as compared to a baseline value for that parameter hasbeen shown to be an early indicator of rejection of a transplantedheart. By careful monitoring of this heart signal parameter, the numberof periodic biopsies of heart tissue as an indicator of transplantrejection can be greatly reduced which provides a significant costsavings as well as a reduction in the myocardial scar tissue created byeach biopsy.

As previously stated, the tracker system includes a device called acardiotracker for processing and recording patient heart electricalsignals, a physician's programmer and an external alarm system. In thepreferred embodiment of the present invention, the cardiotracker isimplanted along with the leads that have electrodes that can sense theheart's electrogram. In an alternative embodiment, the cardiotrackerincluding the electrodes could be external but attached to the patient'sbody. Although the present invention (as described herein) in most casesrefers to the preferred embodiment of an implanted cardiotracker whichcan process electrogram data from implanted electrodes, the techniquesdescribed are equally applicable to an alternative embodiment where anexternal cardiotracker processes electrocardiogram data fromappropriately placed skin surface electrodes.

In the preferred embodiment of the cardiotracker, either or bothsubcutaneous electrodes or electrodes located on a pacemaker type rightventricular or atrial leads can be used. It is also envisioned that oneor more electrodes may be placed within the superior vena cava or othervessels of the circulatory system. One version of the implantedcardiotracker device using subcutaneous electrodes would have anelectrode located under the skin on the patient's left side. This couldbe best located between 2 and 20 inches below the patient's left armpit. The cardiotracker case acting as the indifferent electrode wouldtypically be implanted like a pacemaker under the skin on the upper leftside of the patient's chest. Still another version of the cardiotrackercould utilize epidural electrodes attached externally to the heart. Thisattachment of epidural electrodes to the exterior surface of the heartfrom an epidural lead could take place during the surgery for atransplanted heart.

The physician's programmer is used to program the cardiotracker withrespect to any or all of its diagnostic, detection, alarming andalerting functions. The physician's programmer is also used to retrieveand analyze recorded electrogram segments and other processed heartsignal data from the cardiotracker memory.

Such processed heart signal parameter data includes histograms andstatistical data that can be used to identify changes in cardiovascularcondition over time periods of days, weeks, months or even years. Thehistogram data can be analyzed by the patient's physician using analysistools provided in the physician's programmer. The histogram and/oraverage value data can also be compared against preset thresholds thatare programmed into the cardiotracker. If the thresholds are exceeded,the cardiotracker can activate internal and/or external alarm means foralerting the patient to seek medical attention.

Of particular importance is the ability of the histograms in thecardiotracker to track QRS complex voltage amplitude (or simply the QRSvoltage) on a daily basis. While QRS complex peak-to-peak voltage is thepreferred measurement used for QRS voltage, other signal amplitudes suchas PQ segment to R height or S wave amplitude are also envisioned. Acurrent publication “Clinical Heart Transplantation Without RoutineEndomyocardial Biopsy” by Warnecke et al in The Journal of Heart andLung Transplantation showed that intramyocardial electrogram (IMEG)recordings made with a cardiac pacemaker have the potential to replaceendomyocardial biopsy (EMB) as a diagnostic means to detect transplantrejection. Specifically, Warnecke et al showed that an 8% decline inelectrogram voltage provided the best sensitivity and specificity as theindicator of potential acute moderate allograft rejection.

While pacemakers are not designed to collect weeks or months worth ofstatistical data on electrogram voltage variations, the presentinvention cardiotracker and tracker system is ideally suited for thatpurpose. A daily histogram stored in cardiotracker memory which tracksthe electrogram voltage for every beat analyzed, (e.g., 3 to 12 beatsevery 30 seconds) can provide the data needed to identify potentialtransplant rejection without the need for endomyocardial biopsy. Thehistogram data would be downloaded to the tracker system's physician'sprogrammer for analysis allowing the medical practitioner to identify adrop in electrogram voltage indicative of transplant rejection.Specifically, a decrease in the average value of a multiplicity ofrecently measured QRS voltages compared to a baseline QRS voltage takenwhen the transplanted heart was not being rejected can be used by thecardiotracker to detect the early rejection of a transplanted heart.This detection can also be used to initiate a patient alert warningsignal to advise the patient to seek medical attention. By changingmedications as to type or amount, the rejection of the heart transplantcan be reversed and the patient's life can be saved. It also may bedesirable that the cardiotracker or tracker system programmer be capableof calculating the average (i.e., mean or median) and standard deviationof the distribution of the multiplicity of measured QRS voltagescaptured by a histogram data storage technique. For example a reductionof greater than 8% of the daily mean QRS voltage compared to a baselinevalue for this parameter could be an important indicator of transplantrejection. It is also envisioned that the average QRS voltage over apreset data collection time period (e.g., a day) could be collected by acardiotracker without the need for a histogram.

The cardiotracker histogram capability could also track electrogramsegment voltages as a function of heart rate creating two or morehistograms per day where each histogram represents the distribution ofQRS voltage for every beat in a pre-specified heart rate range.Furthermore, the cardiotracker could be programmed to record the QRScomplex voltage only during a limited time period. It may be preferableto select a time period when the patient would normally be sleeping suchas from midnight to 5 AM.

Similar to the cardiosaver device described by Fischell et al in U.S.Pat. Nos. 6,112,116, 6,272,379 and 6,609,023, which are incorporatedherein by reference, the cardiotracker can detect an acute change in thepatient's electrogram that is indicative of a cardiac event, such as anacute myocardial infarction, within five minutes after it occurs andthen automatically warn the patient that the event is occurring. Toprovide this warning, the tracker system includes an internal alarmsub-system (internal alarm means) within the cardiotracker and/or anexternal alarm system (external alarm means). In the preferredembodiment, the cardiotracker communicates with the external alarmsystem using a wireless radio-frequency (RF) signal. It is envisionedthat the external alarm system of the tracker system would havecapabilities equivalent to those described by Fischell et al in U.S.Pat. Nos. 6,112,116, 6,272,379 and 6,609,023.

As in the Fischell et al devices as previously described, it isenvisioned that there would be at least two types of alarms: amajor/critical event alarm (an “EMERGENCY ALARM”) signaling thedetection of a major cardiac event (e.g., a heart attack which is anAMI) and the need for immediate medical attention, and a less medicallysignificant alert (a “SEE DOCTOR ALERT” or alarm) signaling thedetection of a less serious condition that is not life threatening suchas exercise induced ischemia resulting from a stenosis that is limitingblood flow in a coronary artery. Detection of a decreased QRS voltageindicative of the rejection of a transplanted heart could mostappropriately be indicated by a SEE DOCTOR ALERT because this is not anemergency situation but rather one which should inform the patient tosee a doctor as soon as convenient.

It is also envisioned that the external alarm system of the trackersystem would have capabilities equivalent to that described by Fischellet al in U.S. Pat. Nos. 6,112,116, 6,272,379 and 6,609,023.

Techniques to capture electrogram data and heart signal parameter datacomputed from electrograms over days, weeks or months are importantbecause, as discussed above, some of the processes of heart malfunctionare gradual and it is desirable to detect and treat such conditionsbefore the onset of an acute event such as an AMI or ventricularfibrillation or the complete rejection of a transplanted heart. Limitingthe amount of memory and electrical power needed in the implantedcardiotracker to collect, store and analyze the electrogram data lookingfor trends is especially important in implantable and portable systems.

The present invention cardiotracker will compute the value of one ormore heart signal parameters for each of a multiplicity of beats of theelectrogram. These values will be stored in memory for a first timeperiod which is defined as the “data collection time period.” Thecardiotracker would typically store these values of the one or moreheart signal parameters for a multiplicity of data collection timeperiods over a second time period which is defined as the “collecteddata retention time period.” The cardiotracker would typically computeextracted heart signal parameters (e.g., the mean or median value)extracted from the heart signal parameter values stored in memory duringeach data collection time period. The cardiotracker would typicallystore the values of extracted heart signal parameters for a third timeperiod defined as the “extracted data retention time period.” In thepreferred embodiment of the present invention, the values of the one ormore heart signal parameters stored during the data collection timeperiod would be stored as a histogram or histograms.

The present invention cardiotracker can track any combination of thefollowing heart signal parameters:

1. ST segment voltage

2. ST deviation (ST segment amplitude—PQ segment amplitude for a singleheart beat),

3. R-R interval (time period between successive R waves),

4. R-R interval variability,

5. R peak height,

6. R wave width

7. QRS voltage,

8. QRS width,

9. RS width,

10. T wave width and/or amplitude,

11. T wave alternans, and

12. QRS shift (a recent average value of QRS voltage over a datacollection time period minus the baseline QRS voltage where baseline QRSvoltage is the average value of the QRS voltage for a multiplicity ofheart beats at a time when the heart of a heart transplant patient isnot undergoing rejection)

The present invention cardiotracker can also count arrhythmia relatedevents (that are not heart signal parameters) including:

a) incidence of PACs or PVCs

b) PVC beats per electrogram segment,

c) occurrences of two consecutive beats that each have a PVC,

d) the incidence and duration of episodes of ventricular tachycardia,

e) occurrences of three consecutive PVCs and/or

f) the incidence and time duration of episodes of atrial fibrillation.

Some of these data will be predictive of ventricular fibrillation. Forexample, if there is a change in the frequency of beats with a heartsignal parameter that is indicative of a forthcoming episode ofventricular fibrillation, then certain medication may be prescribed oran implantable cardioverter defibrillator (ICD) could be implanted.

In one preferred embodiment of the present invention cardiotracker, theabove mentioned heart signal parameters and/or counts of arrhythmiarelated events are tracked using a histogram technique.

The dictionary defines a histogram as a “representation of a frequencydistribution by means of rectangles whose widths represent classintervals and whose areas are proportional to the correspondingfrequencies”. The present invention cardiotracker is designed to createhistograms to track the frequency distribution of beats (number of beatsin a preset time period) having heart signal parameter levels within amultiplicity of pre-specified ranges (class intervals). Such a histogramcould be displayed by the physician's programmer as a bar chart (acollection of rectangles) where the width of each bar represents asingle pre-specified range (class interval) of a heart signal parameterand the area of the bar (height.times.width) is proportional to thenumber of beats (corresponding frequency) in that range of the heartsignal parameter. The preferred embodiment of the present invention usesa uniform width (pre-specified range) for each bar and has the height ofthe bar equal to the number of beats in the data collection time periodhaving that one heart signal parameter within that pre-specified range.As an example, in the heart rate range of 50 to 80 beats per minute(bpm), the height of a particular bar could indicate that in the datacollection time period of 24 hours, there were 3,005 beats having ameasurement of QRS voltage between 96% and 98% of the baseline QRSvoltage that was measured during a period of 24 hours at 10 days afterthe heart transplant surgery when biopsy showed no indications ofrejection. In a preferred embodiment of the present invention the QRSvoltage range of 96% to 98% of baseline would also be expressed as thepercent deviation from baseline QRS voltage of −4% to −2%.

The histograms of the present invention can be used to aid the medicalpractitioner in determining if a patient is developing a potentiallydangerous heart condition. As far as the detection of ischemia(including detection of AMI) is concerned, the tracker system asdescribed herein could accurately be called an “Ischemia ManagementSystem” or IMS. The use of such histograms will be clarified with theassistance of FIGS. 6A, 6B, 7A and 7B as provided below in the DETAILEDDESCRIPTION OF THE INVENTION.

In addition, the present invention cardiotracker could provide a set ofhistograms where each histogram represents a range of a first heartsignal parameter and the class intervals of each histogram representpre-specified ranges of a second heart signal parameter. For example, afirst heart signal parameter would be the R-R interval for the beat andthe second heart signal parameter would be the ST deviation. It is alsoenvisioned that the cardiotracker would contain a multiplicity ofhistogram sets where each set would represent the data collected from adifferent time period (e.g., if the data collection time period is aday, then 7 sets are needed for a week and that week would be thecollected data retention time period).

Furthermore, the implanted cardiotracker can process the histogram(s) tocompute extracted histogram data such as:

1. the median ST deviation for each histogram,

2. the histogram bin having the highest value for a specific parameter,

3. The mean value of ST deviation for each histogram,

4. the standard deviation of the histogram distribution with respect tothe highest value bin or with respect to the mean or median,

5. The number of beats per day per histogram exceeding a pre-specifiedthreshold of ST deviation,

6. The moving average over two or more data collection time periods ofany of items 1 through 5,

7. The median of the QRS or RS width histogram, and

8. The average (mean and/or median) QRS voltage over a pre-specifiedtime period and/or within a certain range of heart beats per minute.

9. The QRS shift which is the average QRS voltage over a data collectiontime period compared to the baseline QRS voltage. QRS shift is typicallythe average QRS voltage given as a percentage deviation from thebaseline QRS voltage.

If number 5 above is used, suggested values for each pre-specified STdeviation histogram threshold could be calculated by the programmerbased on previously collected histogram data.

The extracted histogram data can then be compared by the cardiotrackerwith a detection threshold. If the threshold is exceeded, thecardiotracker can take one or more actions including alerting thepatient by means of a SEE DOCTOR ALERT. It is also envisioned that thecardiotracker could compare changes in extracted data between two timeperiods to detect a change that warrants alerting the patient.

Examples of use of these histograms for the present invention are asfollows:

1. For each beat it processes, the cardiotracker would typically computethree heart signal parameters, the ST deviation (i.e., average STsegment signal level minus average PQ segment signal level), the QRSvoltage and the R-R interval which is the time between heart beats whoseinverse is a measure of heart rate. The QRS voltage might be computedonly during a programmed period each day (e.g., during sleep) while theST deviation would typically be monitored all the time.

2. a. The cardiotracker memory would have a current section containing aset of five ST deviation histograms, where each of the five histogramscorresponds to a different range of heart rate (i.e., five different R-Rintervals). Each ST deviation histogram has (for example) 25 bins whereeach bin acts as a counter for the number of beats having an STdeviation within a specific range. That specified range has previouslybeen termed the “class interval”. An example of the specific range orclass interval for ST deviation might be between −7.5% and −2.5% of theamplitude of the average baseline ST deviation taken at approximatelythe same time on the prior day

b. The cardiotracker memory would also have a current section containingtwo or three QRS voltage histograms, where each of the QRS voltagehistograms corresponds to a different range of heart rates (i.e.,different R-R intervals). Each QRS voltage histogram has (for example)25 bins where each bin acts as a counter for the number of beats havinga QRS voltage within a specific range.

3. The value of the R-R interval computed in (1) above will be used bythe cardiotracker to select one of the five ST deviation histograms (2a)into which the ST deviation data for the beat is placed, and one of theQRS voltage histograms (2b) into which the QRS voltage data for the beatis placed. That is, an R-R interval of 1.0 second corresponds to a heartrate of 60 bpm. Therefore, if the R-R interval is 1.0 second, data on aparticular heart signal parameter would be placed in that specifichistogram for heart rates between 50 and 80 bpm.

4. The value of ST deviation computed in (1) above will then be used topick and increment by one, one of the bins within the selected STdeviation histogram where the value of ST deviation of the beat lieswithin the range of ST deviation associated with that specific bin. Thevalue of QRS voltage computed in (1) above will then be used to pick andincrement by one, one bin within the selected QRS voltage histogramwhere the value of QRS voltage of the beat lies within the range of QRSvoltage associated with that specific bin.

It is also envisioned that instead of a single histogram per datacollection time period, there might be a set of histograms allowing thecardiotracker to track the first heart signal parameter (e.g., thoselisted above) for different ranges of a second heart signal parameter.For example, QRS voltage might be tracked in a set of three differenthistograms where each of the three histograms in the set corresponds toa different range of R-R interval or heart rate. Furthermore, these datacan be tracked where each histogram (or histogram set) represents a timeperiod as short as a minute to as long as several years. Similarly, manyhistograms or histogram sets corresponding to successive data collectiontime periods may be stored in the cardiotracker and/or programmer toallow the physician to follow the long term cardiovascular condition ofthe patient.

In a preferred embodiment of the present invention, a multiplicity ofhistogram sets would track the frequency distribution of beats withrespect to two heart rate parameters where each set would correspond toone day. Eight sets would be contained in memory to provide one set forthe current day and seven sets corresponding to the previous seven days.

Additional memory for extracted histogram data would hold basic and/orprocessed extracted data for each histogram in each set for each day foras long as a year. This provides tremendous data compression. Forexample, with only 2 kilobytes of memory, the cardiotracker memory couldstore any of the following types and amounts of data:

1. 10 seconds of electrogram data at 200 samples per second, or

2. 8 days of histogram data in 5 different heart rate ranges with 25bins per histogram, or

3. 6 months of the average value of a heart signal parameter (viz., theaverage value of the QRS voltage within a particular range of heartrates)., and number of beats in each day's histograms from (2) above.

For the purpose of this disclosure, the term “data collection timeperiod” is defined as the time during which the cardiotracker will beupdating a histogram or histogram set. The data collection time periodcould be as short as a minute and as long as many months. Ideally,collection on a daily basis would provide important information andwould minimize effects from daily cycles. A data collection time periodof less than an hour would be useful to collect ST deviation vs. heartrate data during a stress test in the doctor's office. The datacollected during such a stress test could be compared to earlier stresstests using analysis tools built into the physician's programmer of thetracker system. In this way the doctor could detect an increased levelof coronary ischemia which may be caused by progressive narrowing of oneor more coronary arteries.

The “collected data retention time period” is hereby defined as the timeperiod over which a histogram or histogram set is stored incardiotracker memory before it is overwritten with new data. For exampleif the data collection time period is one day and there are 8 sectionsof histogram memory (each corresponding to a day), then one section willbe the current day with histogram stored from the 7 previous days thusthe collected data retention time period is 7 days. The “extracted dataretention time period” is similarly defined as the time period overwhich the extracted histogram data is stored in cardiotracker memorybefore it is overwritten with new data. For example, if the extractedhistogram data (median ST deviation and number of counts) are extractedat the end of each day from that day's histogram, and each day's valueof extracted data is stored in cardiotracker memory for 6 months beforeit is overwritten with new data, then the extracted data retention timeperiod is 6 months.

Important aspects of the present invention are the techniques used bythe physician's programmer to display the collected histogram data toallow a physician to clearly see trends in his patient's cardiovascularcondition. These displays include:

1. a screen including bar charts separately showing each of thehistograms in a set of histograms for one or more data collection timeperiods (e.g., one or more days). For example, the five ST deviationhistograms corresponding to five different heart rate ranges form a setof histograms and QRS voltage histograms for two or three differentheart rate ranges form a set of QRS voltage histograms,

2. a screen that shows line graphs combining all of the histograms in aset of histograms for one or more data collection time periods whereeach histogram in the set is represented by a different line, Each linebeing either a different pattern (e.g. solid line, dashed line, dottedline, etc.) or a different color for a line.

3. a screen including the line graphs of item 2 for more than one datacollection time period where a typical data collection time period isone day, and

4. a screen including a line graph of one or more types of extractedhistogram data as a function of time (e.g., the QRS shift) plotted eachday for a period of 6 months where the 6 months is the extracted dataretention time period).

The physician's programmer would also be used by the physician to defineor select the heart signal parameters that will be tracked using thehistogram technique. It is also envisioned that the physician'sprogrammer will be able to process the histogram data downloaded fromthe patient's cardiotracker to suggest detection thresholds for thedetection by the cardiotracker of future cardiac events that warrantpatient alerting or alarming.

An important part of the concept of the present invention is thecomparison of a recent value for some heart signal parameter with abaseline value for that parameter that was measured at a prior time. Thebaseline value would typically be an average value of the heart signalparameter collected over a pre-specified period of time, e.g., the datacollection time period.

For example, while it is envisioned that the cardiotracker might measurethe QRS voltage for each beat and use the actual measured QRS voltagevalues to populate QRS voltage histograms, a preferred embodiment of thepresent invention would track the QRS voltage for each beat as apercentage of baseline QRS voltage or preferably as the percentdeviation (change) from the baseline QRS voltage. In a preferredembodiment of the present invention, the histograms would thereforetrack the percentage deviation from baseline QRS voltage. Similarly, theaverage QRS voltage for each data collection time period would betracked as a percentage deviation from the baseline QRS voltage. AverageQRS voltage for each data collection time period is an example ofextracted histogram data that would be stored in the extracted histogramdata memory of the cardiotracker. For example, the cardiotracker couldcalculate the baseline QRS voltage being the average value of the QRSvoltage for one day at a time after a heart was transplanted into ahuman subject when traditional medical testing showed that the heart isnot being rejected. This would serve as the “baseline QRS voltage”against which all future QRS voltage measurements would be compared. Theuseful concept here being that a significant decline of the current QRSvoltage compared to the baseline QRS voltage would indicate that thetransplanted heart is being rejected.

For example, each day after the baseline QRS voltage is obtained, thevalue of the day's average QRS voltage (either as measured or as apercent deviation from the baseline QRS voltage) would be placed in thecomputer memory of the cardiotracker. This would be the “recent” averageQRS voltage. The cardiotracker would be designed to detect transplantrejection when the deviation between the recent average QRS voltagecompared to the baseline QRS voltage exceeds a preset threshold. Thus,if the recent daily average QRS voltage was less than the baseline QRSvoltage by more than (let us say) 8%, the cardiotracker would detectrejection. If enabled, the patient alerting function of thecardiotracker would then initiate a SEE DOCTOR ALERT to be triggeredfrom either or both an internal alarm means and/or an external alarmmeans. This alarm would alert the patient to seek medical attention in atimely manner, hopefully, to save the patient's life.

While it may be sufficient to detect transplant rejection when thedeviation of average daily QRS voltage as compared to the baseline QRSvoltage exceeds a preset threshold for a single day, it may be morereliable to require that the threshold be exceeded for two or moresequential days.

Thus it is an object of this invention is to have a tracker systemincluding a cardiotracker designed to track slow changes in thecondition of the patient's heart.

Another object of the present invention is to have a tracker systemincluding a cardiotracker designed to track one or more heart signalparameters through the use of stored histograms.

Still another object of the present invention is to have a cardiotrackercapable of comparing basic or processed extracted histogram data with aphysician-set threshold and alerting the patient when that threshold iscrossed.

Still another object of the present invention is to have a cardiotrackerthat can calculate a moving average of extracted histogram data overrelevant time periods and use the moving average to track the conditionof the patient's heart.

Still another object of the present invention is to have the physician'sprogrammer process downloaded histogram and extracted histogram datafrom the cardiotracker to suggest detection thresholds for acute cardiacevent detection by the cardiotracker.

Still another object of the present invention is to have thecardiotracker determine average values for QRS voltage over a datacollection time period and also have the capability to provide a SEEDOCTOR ALERT if that average value of the QRS voltage deviates from abaseline QRS voltage by more than a preset amount for one or moresequential data collection time periods.

Yet another object of the present invention is to have a cardiotrackerstore QRS voltage as a percentage of the baseline QRS voltage.

Yet another object of the present invention is to have a cardiotrackerstore QRS voltage as a percentage deviation from the baseline QRSvoltage.

Yet another object of the present invention is to have a cardiotrackercompute the average QRS voltage over a data collection time period as apercentage deviation from the baseline QRS voltage, which percentagedeviation is the QRS shift.

These and other objects and advantages of this invention will becomeobvious to a person of ordinary skill in this art upon reading of thedetailed description of this invention including the associated drawingsas presented herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a tracker system for the detection of a cardiac eventand for warning the patient that a cardiac event is occurring.

FIG. 2 is a plan view of an implantable cardiotracker showing thecardiotracker electronics module and two electrical leads each havingone electrode.

FIG. 3 is a block diagram of the cardiotracker.

FIG. 4 illustrates a normal electrogram pattern with a set of typicalheart signal parameters.

FIG. 5 is a block diagram showing the structure of the histogram datamemory.

FIG. 6A is an example of a programmer display screen showing a set ofhistograms for ST deviation for a single data collection time period(viz., one day), where each histogram corresponds to a different heartrate range.

FIG. 6B is an example of a programmer display screen showing a set ofhistograms for the percent deviation of the QRS voltage from a baselineQRS voltage for a single data collection time period (viz., one day),where each histogram corresponds to a different heart rate range.

FIG. 7A is an example of a programmer display screen showing STdeviation histograms for three different days, where each frequency plotshows 5 different heart rate ranges on one graph with multiple lines.

FIG. 7B is an example of a programmer display screen showing histogramsfor the percent deviation of the QRS voltage from the baseline QRSvoltage for three specific days and for three different ranges of heartrate.

FIG. 8A is an example of the programmer display screen showing agraphical representation of the 5 day moving average of the dailyaverage ST deviation for each of five heart rate ranges over a period of26 weeks.

FIG. 8B is an example of the programmer display screen showing agraphical representation of the percent deviation of the daily medianQRS voltage from the baseline QRS voltage for each of three heart rateranges over a period of 26 weeks.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an example of a tracker system 10 including animplanted cardiotracker 5 and external equipment 7. The cardiotracker 5includes electrical wire leads 12 and 15 and a battery-poweredelectronics module contained within a metal case 11. The cardiotracker 5can track the patient's cardiovascular condition over extended periodsof time. The cardiotracker 5 can also detect acute cardiac eventsincluding acute myocardial infarction and arrhythmias and warn thepatient when such an event occurs. The cardiotracker 5 can also trackslowly changing cardiac functions such as day-to-day changes in QRSvoltage that can be indicative of the rejection of a transplanted heart.The cardiotracker 5 can record the patient's electrogram signal forlater review by a medical practitioner. The cardiotracker 5 can capturehistogram-based historical representations of one or more heart signalparameters for later analysis and review by a medical practitioner. Thecardiotracker 5 can also send out wireless signals 53 to and receivewireless signals 54 from the external equipment 7. The functioning ofthe cardiotracker 5 will be explained in greater detail with theassistance of FIGS. 2, 3, 4 and 5.

The cardiotracker 5 has two leads 12 and 15 that have one or moreelectrical conductors (wires) with surrounding insulation. The lead 12is shown with two electrodes 13 and 14. The lead 15 has subcutaneouselectrodes 16 and 17. In fact, the cardiotracker 5 could utilize as fewas one lead or as many as three and each lead could have as few as oneelectrode or as many as eight electrodes. Furthermore, electrodes 8 and9 could be placed on the outer surface of the case 11 without any wireleads extending from the cardiotracker 5.

The lead 12 in FIG. 1 could advantageously be placed through thepatient's vascular system with the electrode 14 being placed into theapex of the right ventricle. The lead 12 with electrode 13 could beplaced in the right ventricle or right atrium or the superior vena cavasimilar to the placement of leads for pacemakers and implantablecardioverter defibrillators (ICDs). The metal case 11 of thecardiotracker 5 could serve as an indifferent electrode with either orboth electrodes 13 and/or 14 being active electrodes. It is alsoconceived that the electrodes 13 and 14 could be used as bipolarelectrodes. Alternately, the lead 12 in FIG. 1 could advantageously beplaced through the patient's vascular system with the electrode 14 beingplaced into the apex of the left ventricle. The electrode 13 could beplaced in the left atrium.

The lead 15 could advantageously be placed subcutaneously at anylocation where the electrodes 16 and/or 17 would provide a goodelectrogram signal indicative of the electrical activity of the heart.Again for this lead 15, the case 11 of the cardiotracker 5 could be anindifferent electrode and the electrodes 16 and/or 17 could be activeelectrodes or electrodes 16 and 17 could function together as bipolarelectrodes. The cardiotracker 5 could operate with only one lead and asfew as one active electrode with the case 11 of the cardiotracker 5being an indifferent electrode. The tracker system 10 described hereincan readily operate with only two electrodes. It is also envisioned thatthe lead 15 could be an epicardial lead with the electrode 17 beingfirmly attached to the heart muscle from outside of the patient's heartand the electrode 13 being implanted elsewhere within the patient'sbody.

One embodiment of the cardiotracker device 5 using subcutaneous lead 15would have the electrode 17 located under the skin on the patient's leftside. This could be best located between 2 and 20 inches below thepatient's left arm pit. The cardiotracker case 11 could act as theindifferent electrode and would typically be implanted under the skin onthe upper left side of the patient's chest. Alternately, both electrodes8 and 9 could, like the Medtronic Reveal™, be located on the surface ofthe cardiotracker case 11.

FIG. 1 also shows the external equipment 7 that consists of aphysician's programmer 68 having an antenna 70 and an external alarmsystem 60 including a charger 166 that could be used to charge arechargeable battery (not shown) in the external alarm system 60. Itshould be understood that the external alarm system 60 could also bepowered by a conventional (i.e., non-rechargeable) battery. The externalequipment 7 provides means to interact with the implanted cardiotracker5. These interactions include programming the cardiotracker 5,retrieving data collected by the cardiotracker 5 and handling alarmsgenerated by the cardiotracker 5.

The purpose of the physician's programmer 68 shown in FIG. 1 is to setand/or change the operating parameters of the implantable cardiotracker5 and to read out data stored in the memory of the cardiotracker 5 suchas stored electrogram segments, histograms and extracted histogram data.This would be accomplished by transmission of a wireless signal 54 fromthe programmer 68 to the cardiotracker 5 and receiving of telemetry bythe wireless signal 53 from the cardiotracker 5 to the programmer 68.When a laptop computer is used as the physician's programmer 68, itwould require connection to a wireless transceiver for communicatingwith the cardiotracker 5. Such a transceiver could be connected via astandard interface such as a USB, serial or parallel port or it could beinserted into the laptop's PCMCIA card slot. The screen on the laptopphysician's programmer 68 would be used to provide guidance to themedical practitioner in communicating with the cardiotracker 5. Also,the screen could be used to display both real time and storedelectrograms that are read out from the cardiotracker 5 as well ashistograms and extracted data based on any one of several heart signalparameters.

In FIG. 1, the external alarm system 60 has a patient operated initiator55, an alarm disable button 59, a panic button 52, an alarm transceiver56, a speaker 57, a modem 165 and an antenna 161. The modem 165 allowsdata transmission to and from medical services 67 via the communicationlink 65. It is also envisioned (but not shown in FIG. 1) that theexternal alarm system 60 could include a microphone and associatedelectronics for two-way voice communication with the medical services67.

If a cardiac event is detected by the cardiotracker 5 or the long termcardiovascular tracked data has exceeded a programmed limit, an alarmmessage is sent by a wireless signal 53 to the alarm transceiver 56 viathe antenna 161. When the alarm message is received by the alarmtransceiver 56, a signal 58 is then sent to the loudspeaker 57. Thesignal 58 will cause the loudspeaker 57 to emit an external audio alarmsignal 51 to warn the patient that an event has occurred. Examples ofexternal alarm signals 51 include a periodic buzzing, a sequence oftones and/or a speech message that instructs the patient as to what ishappening and what actions should be taken. Furthermore, the alarmtransceiver 56 can, depending upon the nature of the signal 53, can sendan outgoing signal over the link 65 to contact emergency medicalservices 67. When the detection of an acute myocardial infarction orother life threatening cardiac event (e.g., tachycardia) is the cause ofthe alarm, the alarm transceiver 56 could automatically notify medicalservices 67 that a serious cardiac event has occurred and an ambulancecould be sent to treat the patient and to bring him to a hospitalemergency room or directly to a catheterization laboratory.

If communication with medical services 67 is enabled and a cardiac eventalarm is sent within the signal 53, the modem 165 will establish thedata communications link 65 over which a message will be transmitted tothe medical services 67. The message sent over the link 65 may includeany one, a combination of several or all of the following informationtypes: (1) a specific patient is having an acute myocardial infarctionor other cardiac event, (2) the patient's name, address and a briefmedical history, (3) a map and/or directions to where the patient islocated (using the GPS satellite or cellular location means is alsoenvisioned), (4) the patient's stored electrogram including baselineelectrogram data and the specific electrogram segment that generated thealarm (5) continuous real time electrogram data, and (6) a prescriptionwritten by the patient's personal physician as to the type of treatmentand/or the amount of drug to be administered to the patient in the eventof a specific cardiac event. If the medical services 67 include anemergency room at a hospital, information can be transmitted that thepatient has had a cardiac event and should be on his way to theemergency room. In this manner the medical practitioners at theemergency room and/or a catheterization laboratory could be prepared forthe patient's arrival.

Just as the ONSTAR™ service will respond to help a driver immediatelyafter a car's air bags deploy, so might the medical services 67 respondto the patient upon receipt of information that a serious cardiac eventhas occurred. Such a serious cardiac event would cause an EMERGENCYALARM signal to be initiated by the internal alarm means in thecardiotracker and (if within range) an external alarm would sound fromthe external alarm system 60. Based on the patient's cardiac event andprior instructions from the patient's physician, the medical servicespersonnel can instruct the patient and summon appropriate help.

The purpose of the patient operated initiator 55 is to give the patientthe capability for initiating transmission of captured electrogramsegments and histogram data from the cardiotracker 5, through theexternal alarm system 60, to a medical practitioner at the medicalservices 67. This will enable one or more electrogram segments to bedisplayed for a medical practitioner. The alarm disable button 59 can beused by the patient to turn off the internal alarm signal generatedwithin the cardiotracker 5 and/or turn off the external alarm signal 51played through the speaker 57. If the alarm disable button is notpressed, either or both the internal and external alarms would continuefor a preset period of time such as 15 minutes. A reminder alarm signalmight then be triggered at some later time (e.g., 2 to 5 hours later) ifthe patient has not turned off the alarms by means of the alarm disablebutton 59.

The patient might press the panic button 52 in the event that thepatient feels that he is experiencing a cardiac event even if there isno alarm signal from either the internal or external alarm means. Thepanic button 52 will initiate the transmission from the cardiotracker 5to the external alarm system 60 via the wireless signal 53 of bothrecent and baseline electrogram segments. Also, following the use of thepanic button 52, the tracker system 10 can be programmed to transmit thelast set of histograms tracking a particular aspect of the patient'scardiovascular condition. In addition, an analysis of the histogramdata, for example, the 5 day moving average of a heart signal parameter(e.g., ST deviation) over the last week or month, may be transmitted tomedical practitioners at the medical services 67 to allow them to seetrends in the patient's cardiovascular condition. The external alarmsystem 60 will then retransmit these data via the link 65 to medicalservices 67 where a medical practitioner will view the data. The medicalpractitioner remotely located at the medical services 67 could thenanalyze the data and call the patient back to offer advice as to whetherthis is an emergency situation or the situation could be routinelyhandled by the patient's personal physician at some later time.

FIG. 2 is a plan view of the cardiotracker 5 having a metal case 11 anda plastic header 20. The case 11 contains the battery 22 and theelectronics module 18. This type of package is well known forpacemakers, implantable defibrillators and implantable tissuestimulators. Electrical conductors placed through the plastic header 20connect the electronics module 18 to the electrical leads 12 and 15,which have respectively electrodes 14 and 17. The lead electrodes 13 and16 and the on-case electrodes 8 and 9 of FIG. 1 are not shown in FIG. 2.It should also be understood that the cardiosaver 5 can function withonly two electrodes, one of which could be the case 11. All thedifferent configurations for electrodes shown in FIGS. 1 and 2, such asthe electrodes 8, 9, 13, 14, 16 or the metal case 11 are shown only toindicate that there are a variety of possible electrode arrangementsthat can be used with the cardiosaver 5.

On the metal case 11, a conducting disc 31 mounted onto an insulatingdisc 32 can be used to provide a subcutaneous electrical tickle to warnthe patient with a SEE DOCTOR ALERT or an EMERGENCY ALARM or the disc 31could act as an independent electrode for sensing the patient'selectrogram. Alternatively, the electrode 8 or the electrode 9 of FIG. 1could be used as a sensing electrode for the electrogram.

FIG. 3 is a block diagram of the cardiotracker 5 with battery 22. Theelectrodes 14 and 17 connect with wires within the leads 12 and 15respectively to the amplifier 36 that is also connected to the case 11acting as an indifferent electrode. As two or more electrodes 14 and 17are shown here, the amplifier 36 would be a multi-channel amplifier. Ifonly one electrode was used, the amplifier would be a single channelamplifier. The amplified electrogram signals 37 from the amplifier 36are converted to digital signals 38 by the analog-to-digital converter41. The digital electrogram signals 38 are buffered in theFirst-In-First-Out (FIFO) memory 42. A processor shown as the centralprocessing unit (CPU) 44 coupled to memory means shown as the RandomAccess Memory (RAM) 47 can process the digital electrogram data 38stored within the FIFO 42 according to the programming instructionsstored in the program memory 45. This programming (i.e., software)enables the cardiotracker 5 to detect the occurrence of cardiac eventssuch as an acute myocardial infarction.

A clock/timing sub-system 49 provides the means for timing specificactivities of the cardiotracker 5 including the absolute or relativetime stamping of detected cardiac events. The clock/timing sub-system 49can also facilitate power savings by causing components of thecardiotracker 5 to go into a low power standby mode in between times ofelectrogram signal collection and processing. Such cycled power savingstechniques are often used in implantable pacemakers and defibrillators.In an alternative embodiment, the function of the clock/timingsub-system 49 can be provided by a program subroutine run by the centralprocessing unit 44.

In a preferred embodiment of the present invention, the RAM 47 includesspecific memory locations for 3 sets of electrogram segment storage.These are the recent electrogram storage 472 that would store the last 2minutes to 24 hours of recorded electrogram segments so that theelectrogram data for the last day (even if there are no events) or inthe period just before the onset of a cardiac event can be reviewed at alater time by the patient's physician using the physician's programmer68 of FIG. 1. For example, the recent electrogram memory 472 mightcontain eight, 10 second long electrogram segments that were capturedevery 30 seconds over the prior 4 minute time period. The baselineelectrogram memory 474 would also provide storage for baselineelectrogram segments collected at preset times over one or more days.For example, the baseline electrogram memory 474 might contain 24baseline electrogram segments of 10 seconds duration, one from each hourfor the prior 24 hours.

The event memory 476 occupies the largest part of the RAM 47. The eventmemory 476 is not overwritten on a regular schedule as are the currentelectrogram memory 472 and baseline electrogram memory 474 but istypically maintained until read out by the patient's physician with theprogrammer 68 of FIG. 1. When a cardiac event such as excessive ST shiftindicating an acute myocardial infarction is detected by the CPU 44, all(or part) of the entire contents of the baseline and recent electrogrammemories 472 and 474 would typically be copied into the event memory 476so as to save the pre-event data for later physician review. Followingthe occurrence of a cardiac event, post event electrogram data would besaved in the event memory 476 for a preset time period.

The RAM 47 also contains memory sections for programmable parameters 471and calculated baseline data 475. The programmable parameters 471include the upper and lower limits for the normal and elevated heartrate ranges and physician programmed parameters related to the cardiacevent detection processes stored in the program memory 45. Thecalculated baseline data 475 contain detection parameters extracted fromthe baseline electrogram segments stored in the baseline electrogrammemory 474. Calculated baseline data 475 and programmable parameters 471would typically be saved to the event memory 476 following the detectionof a cardiac event. The RAM 47 also includes patient data 473 that mayinclude the patient's name, address, telephone number, medical history,insurance information, doctor's name, and specific prescriptions fordifferent treatments or medications to be administered by medicalpractitioners in the event of different cardiac events.

Finally, the RAM 47 contains histogram data memory 43 whose structure isshown in FIG. 5.

It is envisioned that the cardiotracker 5 could also contain pacemakercircuitry 170 and/or defibrillator circuitry 180 similar to thecardiosaver device described by Fischell et al in U.S. Pat. No.6,240,049.

The alarm sub-system 48 is the internal alarm means that contains thecircuitry and transducers to produce the internal alarm signals for thecardiotracker 5. The internal alarm signal can be a mechanicalvibration, a sound or a subcutaneous electrical tickle.

The telemetry sub-system 46 with antenna 35 provides the cardiotracker 5the means for two-way wireless communication to and from the externalequipment 7 of FIG. 1. The outgoing signal 53 being from thecardiotracker 5 to the external equipment 7 and the incoming signal 54being from the external equipment 7 to the cardiotracker 5. Existingradiofrequency transceiver chip sets such as the CHIPCOM CC1000 or theAsh transceiver hybrids produced by RF Microdevices, Inc. can readilyprovide such two-way wireless communication over a distance of up to 10meters from the patient. It is also envisioned that short rangetelemetry (less than 6 inches) such as that typically used in pacemakersand defibrillators could also be applied to the cardiotracker 5. It isalso envisioned that standard wireless protocols such as Bluetooth and802.11a, 802.11b or 802.11g might be used to allow communication with awider group of externally located peripheral devices.

A magnet sensor 190 could be incorporated into the cardiotracker 5. Animportant use of the magnet sensor 190 is to turn on the cardiotracker 5on just before programming and implantation into a human subject. Thiswould reduce wasted battery life in the period between the times thatthe cardiotracker 5 is packaged at the factory until the time that it isimplanted into the human subject.

FIG. 4 highlights the features of one normal beat 500 of an electrogramsegment and also shows some portions of the prior beat. The beat 500shows typical heart beat wave elements labeled P, Q, R, S and T. Thebeat 500 is defined to be a sub-segment of an electrogram segmentcontaining exactly one R wave and including the P and Q elements beforethe R wave and the S and T elements following the R wave. The R-Rinterval 507 for the beat 500 is defined as the time from the R wavebefore the beat 500 to the R wave of the beat 500. Both the prior R waveand the R wave of the beat 500 are shown in FIG. 4.

For the purposes of detection algorithms, different sub-segments,elements and calculated values related to the beat 500 are herebyspecified. The peak of the R wave of the beat 500 occurs at the timeT.sub.R (509). The PQ segment 501 and ST segment 505 are sub-segments ofthe normal beat 500 and are located in time with respect to the timeT.sub.R (509) as follows:

a. The PQ segment 501 has a time duration D.sub.PQ (506) and startsT.sub.PQ (502) milliseconds before the time T.sub.R (509).

b. The ST segment 505 has a time duration D.sub.ST (508) and startsT.sub.ST (502) milliseconds after the time T.sub.R (509).

The ST segment 505 and the PQ segment 501 are examples of sub-segmentsof the electrogram signal from a patient's heart. The R wave and T waveare also sub-segments. The dashed lines V.sub.PQ (512) and V.sub.ST(514) illustrate the average voltage amplitudes of the PQ and STsegments 501 and 505 respectively for the normal beat 500. The “STdeviation” .DELTA.V (510) of the normal beat 500 is defined as:.DELTA.V(510)=V.sub.ST(514)−V.sub.PQ(512)

The parameters T.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.ST would typicallybe set with the programmer 68 of FIG. 1 by the patient's doctor at thetime the cardiotracker 5 is implanted so as to best match the morphologyof the patient's electrogram signal at a normal (e.g., resting) heartrate.

The R height V.sub.PQR (519) for the beat 500 is defined asV.sub.PQR(519)=V.sub.R(503)−V.sub.PQ(512)

V.sub.PQ (512), V.sub.ST (514), V.sub.R (503), V.sub.PQR (519) and.DELTA.V (510) are examples of per-beat heart signal parameters for thebeat 500.

Although it may be effective to fix the values of start times T.sub.PQ(502) and T.sub.ST (504) and the time durations D.sub.PQ (506) andD.sub.ST (508), it is envisioned that the start times T.sub.PQ andT.sub.ST and the durations D.sub.PQ and D.sub.ST could be automaticallyadjusted by the cardiotracker 5 to account for changes in the R-Rinterval 507 (i.e., changes in the patient's heart rate). If the R-Rinterval 507 increases or decreases, as compared with the R-R intervalfor patient's normal heart rate, it is envisioned that the start timesT.sub.PQ (502) and T.sub.ST (504) and/or the durations D.sub.PQ (506)and D.sub.ST (508) could be adjusted depending upon the R-R interval 507for a specific beat or the average R-R interval for an entireelectrogram segment. A simple technique for doing this would vary thestart times T.sub.PQ and T.sub.ST and the durations D.sub.PQ andD.sub.ST in proportion to the change in R-R interval. For example, ifthe patient's normal heart rate is 60 beats per minute, the R-R intervalis 1 second. At 80 beats per minute the R-R interval is 0.75 seconds, a25% decrease. This could automatically produce a 25% decrease in thevalues of T.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.ST. Alternately, thevalues for T.sub.PQ, T.sub.ST, D.sub.PQ and D.sub.ST could be fixed foreach of up to 20 preset heart rate ranges. In either case, it isenvisioned that after the device has been implanted, the patient'sphysician would, through the programmer 68 of FIG. 1, download from thecardiotracker 5 to the programmer 68, a recent electrogram segment fromthe recent electrogram memory 472 (of FIG. 3). The physician would thenuse the programmer 68 to select the values of T.sub.PQ, T.sub.ST,D.sub.PQ and D.sub.ST for the heart rate in the downloaded recentelectrogram segment. The programmer 68 would then allow the physician tochoose to either manually specify the values of T.sub.PQ, T.sub.ST,D.sub.PQ and D.sub.ST for each heart rate range or have thecardiotracker 5 automatically adjust the values of T.sub.PQ, T.sub.ST,D.sub.PQ and D.sub.ST based on the R-R interval 507 for each beat of anyelectrogram segment collected in the future by the cardiotracker 5. Itis also envisioned that only the start times, T.sub.PQ and T.sub.ST,might be automatically adjusted and the time durations D.sub.PQ andD.sub.ST would be fixed so that the average values of the ST and PQsegments V.sub.PQ (512), V.sub.ST (514), V′.sub.PQ (512′) and V′.sub.ST(514′) would always use the same number of data samples for averaging.

While the simplest method of adjusting the start times T.sub.PQ andT.sub.ST is to adjust them in proportion to the R-R interval 507 fromthe preceding R wave to the R wave of the current beat, a preferredembodiment of the present invention is to adjust the start timesT.sub.PQ and T.sub.ST in proportion to the square root of the R-Rinterval 507 from the preceding R wave to the R wave of the currentbeat. It is also envisioned that a combination of linear and square roottechniques could be used where T.sub.ST and D.sub.ST could be set to beproportional to the square root of the R-R interval while T.sub.PQ andD.sub.PQ could be linearly proportional to the R-R interval.

When pacemaker circuitry 170 is used with the cardiotracker 5, itenvisioned that the start time T.sub.ST and duration D.sub.ST of the STsegment may have different values depending on whether or not the heartis being paced. When the pacemaker is pacing the heart, the ST segmentshifts so as to occur later relative to the start of the R wave ascompared to the position of the ST segment when the pacer is not pacingthe heart. It is also envisioned, that the offset for the start of theST segment may be better measured from the S wave instead of the R whenthe pacemaker is not pacing. The technique of using different timingparameters for start and duration when pacing can be applied to analysisof any sub-segment of the electrogram including the sub-segment thatincludes the T wave peak. When the pacemaker circuitry 170 is used withthe cardiotracker 5, the algorithm for measurement of the ST segment canbe adjusted to respond to either the pacing or no-pacing condition ofthe pacemaker circuitry 170.

An example of a sequence of steps used to calculate the ST deviation 510for the normal beat 500 is as follows:

1. Identify the time T.sub.R (509) for the peak of the R wave for thebeat 500,

2. Calculate the R-R interval 507 and use that value to look up in atable or calculate the values of the start times T.sub.PQ, T.sub.ST andthe time durations D.sub.PQ and D.sub.ST,

3. Average the amplitude of the PQ segment 501 between the times(T.sub.R−T.sub.PQ) and (T.sub.R−T.sub.PQ+D.sub.PQ) to create the PQsegment average amplitude V.sub.PQ (512),

4. Average the amplitude of the ST segment 505 between the times(T.sub.R+T.sub.ST) and (T.sub.R+T.sub.ST+D.sub.ST) to create the STsegment average amplitude V.sub.ST (514), and

5. Subtract V.sub.PQ (512) from V.sub.ST (514) to produce the STdeviation, .DELTA.V (510) for the beat 500.

At preset time intervals during the day the cardiotracker 5 willcalculate the “average baseline ST deviation” .DELTA.V.sub.BASE definedas the average of the ST deviations .DELTA.V (510) for at least twobeats of a baseline electrogram segment. Typically the ST deviation of 4to 8 beats of the baseline electrogram segment will be averaged toproduce the average baseline ST deviation .DELTA.V.sub.BASE which can beused for later comparison with the ST deviation of recent beats toidentify changes indicative of a cardiac event such as an acutemyocardial infarction. Fischell et al in U.S. Pat. No. 6,609,023describe in detail the methods for detecting AMI and exercise inducedischemia.

As (for example) the ST deviation, .DELTA.V (510) or the QRS voltage,V.sub.QRS (511) for each beat is calculated, one or more histogramsstored in the histogram data memory 43 of FIGS. 3 and 5 will beincremented with that specific value of that heart signal parameter.

FIG. 5 is an example of a structure for the histogram data memory 43 ofthe cardiotracker 5 of FIG. 3. The histogram data memory 43 contains twotypes of histogram data, raw histogram data stored in the memorysections 430 through 43N and extracted histogram data stored in theextracted histogram data memory 439. One of the raw histogram datasections 430 through 43N will always be the section currently beingincremented as individual beats are processed by the processor 44 ofFIG. 3 to compute the value of one or more heart signal parameters foreach processed beat. The other histogram sections will usually be thehistograms collected during prior data collection time periods.

In this example, each section 430 through 43N has 5 histograms (e.g.,section 430 has histograms 4301, 4302, 4303, 4304 and 4305). Each of the5 histograms in each section has a multiplicity of bins (e.g., histogram4301 has bins 4301 a, 4301 b through 4301 y). Each bin is a counter thatis typically stored in one to 3 bytes of the histogram data memory 43.

As the cardiotracker 5 processes a beat of the patient's electrogram,one or more heart signal parameters will be measured or computed for thebeat. For each processed beat, the counter value of one bin in one ofthe histograms of the current histogram section will be incremented byone.

The choice of which bin in which histogram is incremented will be basedon two heart signal parameters. The selection of one of the 5 histogramswill be based on the value of a first heart signal parameter and thechoice of which bin is to be incremented will depend upon the value of asecond heart signal parameter. Specifically, a specific histogram willbe selected if the value of the first heart signal parameter is withinthe range of the first heart signal parameter associated with thatspecific histogram. Similarly, a bin within the selected histogram willbe incremented if the value of the second heart signal parameter iswithin the range of the second heart signal parameter associated withthat bin.

For example, if the data collection time period used for tracking aheart signal parameter, like ST deviation, is one day and collected dataretention time period is one week, then N=7 (i.e., section 43N issection 437) and there will be 8 sections 430 through 437 in thehistogram memory 43 with seven sections storing the data for each one ofseven prior days and the eighth section storing the data for the currentday. In this example, each of the five histograms per section correspondto a different range of R-R interval (or heart rate) [the first heartsignal parameter] and each bin within a histogram corresponds to adifferent range of ST deviation [the second heart signal parameter]. Asa further example, section 4301 corresponds to heart rates that arebetween 50 and 80 bpm and each of the bins 4301 a through 4301 y wouldcorrespond to a 5% wide (.+−02.5%) range of ST deviation as a percentageof baseline R height. Furthermore bin 4103 a would correspond to a rangeof ST deviation of −60%.+−02.5% of baseline R height and bin 4301 ywould correspond to a range of ST deviation of +60%.+−02.5% of baselineR height. Therefore the bin 4301 n (not shown) would correspond to arange of ST deviation between +2.5% and +7.5% (i.e., 5%.+−02.5) of theaverage baseline level of ST deviation. This bin 4301 n would have thedata shown as the highest bar of graph 601 in FIG. 6A. In FIG. 6A it isshown that there are a total of 25 bins in each of the histograms601-605 inclusive. These bins run from −60% plus or minus 2.5% to +60%plus or minus 2.5%. The 14.sup.th bin is 4301 n which is +5% plus orminus 2.5% and the 25.sup.th bin in section 4301 is 4301 y which is +60%plus or minus 2.5%. The five different heart rate ranges shown for thehistograms 601 to 605 inclusive of FIG. 6A would (for example)correspond to the sections 4301 to 4305 inclusive of FIG. 5.

It is envisioned that the levels of ST deviation can be representativeof actual voltages (e.g., millivolts) or they may be a normalized valuewith respect to the signal amplitude of the beat or electrogram segment.Examples of such a signal amplitude is the QRS voltage V.sub.QRS (511)or the R wave height above the PQ segment which is V.sub.PQR (519) ofFIG. 4.

In FIG. 5, if section 432 is the present day's current histogram, thensection 431 is from the day before, section 430 from 2 days before, andbecause the data rolls over, 437 (not shown) is the histogram for 3 daysbefore, 436 (not shown) from 4 days before, 435 (not shown) from 5 daysbefore, 434 (not shown) from 6 days before and section 433 (not shown)from 7 days before. For each beat analyzed by the cardiotracker for thecurrent day's histogram, the R-R interval (heart rate) for that beat isused to select one of the histograms 4321 through 4325 and the value ofST deviation computed for that beat will be used to select the bin inthe selected histogram that will be incremented by 1. Further using thelabeling of FIG. 4, assume the R-R interval for the beat just analyzedis within the heart rate range of the first histogram 4321 of thecurrent section 432 and the ST deviation 510 of the beat analyzed is−0.1 millivolts which is −1% of the R height 519. In this case the bincorresponding to a range of ST deviations that includes −1% of R heightwill be incremented by 1. In this way each beat is counted in one bin ofone histogram of the current section, in this case, section 432. Over a24 hour period as the patient's heart rate (R-R interval) goes up anddown, the histograms will track the ST deviation of each beat processedin each of the ranges of heart rate.

At the end of the data collection time period (24 hours in this example)during which section 432 is the current section, the cardiotracker willclear section 433 (the section with the oldest data) of all previouslystored data and make section 433 (now empty) the current section fordata collection. The previous current section 432 now becomes thesection from one day before and is saved until the cycle repeats. On theday following the day where section 437 is the current day, section 430will become the current section.

It is envisioned that before clearing section 433, the cardiotrackermight extract or analyze the data in 433 and save the extracted data inthe extracted histogram data memory 439. For example, the median valueof ST deviation could be calculated for section 432 and that data couldbe time stamped as to the day of the year and placed into the extractedhistogram data memory 439. Alternately, the extracted data placed in theextracted histogram data memory 439 may be calculated for the currenthistogram section 432 at the end of the data collection time periodwhere the section 432 was designated as the current section.

Examples of extracted data for any data collection time period caninclude any one, some or all of the following:

1. number of beats in a histogram exceeding an ST deviation or ST shiftthreshold,

2. average ST deviation or average ST shift,

3. standard deviation of ST deviation or ST shift distribution (mayinclude both positive and negative standard deviation values),

4. total number of beats in the histogram (if there are very few beatsin a particular histogram, using the average and/or standard deviationcould be misleading),

5. ST deviation or ST shift bin with greatest number of beats,

6. the moving average over 2 or more data collection time periods of anyof items 1 through 5 immediately above,

7. the average of the QRS or RS width, and

8. the average QRS voltage.

When the patient's physician downloads the data from the histogram datamemory 43 (of FIG. 3), the histograms for the current data collectiontime period up to the time of download, and the complete histograms forthe previous collected data retention time period can all be viewedusing the physician's programmer 68 of FIG. 1.

Although the examples above used one day per section as the datacollection time period, shorter or longer periods are envisioned.Although 8 sections, (representing 7 days plus a current day's histogramsection) are described above, with sufficient memory, a month (32sections), a year (367 sections) or more of data can be saved in thisformat.

Although 5 histograms per section are described in the example above, itis envisioned that as few as one and as many as 100 could be used tocollect relevant data. There are a number of heart signal parametersincluding QRS width or RS width of the electrogram wave form and R-Rinterval variability indicative of changes in the balance of thepatient's sympathetic and parasympathetic nervous systems that are mostlikely to be tracked in a single histogram per data collection timeperiod. Other heart signal parameters such as ST deviation, ST segmentvoltage, ST shift (ST deviation relative to average baseline STdeviation), T wave height, QRS voltage and/or R wave height may bepreferably tracked with respect to heart rate (determined from R-Rinterval) using multiple histograms per section.

It is envisioned, that the data collection time period could be as shortas a minute and as long as many months. A preferred embodiment uses adata collection time period of one day as collection on a daily basiswould eliminate any affects from daily cycles (i.e., from circadianrhythm). A data collection time period of less than an hour would beuseful to collect ST deviation vs. heart rate data during a stress testin the doctor's office. The data collected during such a stress testcould be compared to earlier tests using analysis tools built into thephysician's programmer 68 of the tracker system 10. Histogram data doesnot require large amounts of data storage. For example, each of the fivehistograms 4321 through 4325 of FIG. 5 might have 25 bins 4321 a, 4321 bthrough 4321 y, with each bin requiring 2 bytes of data storage. Thusonly 50 bytes are needed per histogram and 250 bytes for the entiresection 432. The eight sections would therefore require only 2kilobytes, approximately 7.5 kilobytes would suffice for a month's (30days) data and approximately 90 kilobytes for a year of data. Being ableto store a one week to twelve month history of cardiovascular conditionwithin the cardiotracker would be of tremendous value to cardiologistsin diagnosing the progression of cardiovascular disease. Two byte binsare typically sufficient for a day's data as the cardiotracker isdesigned to only monitor some fraction of the beats (e.g., 10 secondsout of every 30 seconds) and a two byte counter could handle every thirdbeat for 54 hours. If a longer data collection time period than 4 daysis required, three bytes could handle more than year's worth of datawhere a third of all beats are captured. Four bytes per bin would besufficient to count every heart beat for one hundred years.

It is also envisioned that the physician's programmer 68 of FIG. 1 couldinclude the capability to manually clear the data in the currenthistogram. This would allow a “clean slate” for data collection from astress test where, as each beat is analyzed, the ST deviation data buildup is a representation of the patient's cardiovascular condition. It isalso envisioned that a special cardiotracker data collection mode whereevery beat is analyzed could be enabled to collect more data during sucha stress test. If every beat is too high a burden on the cardiotrackerprocessor, then the cardiotracker might process a higher percentage ofbeats than during standard cardiotracker operation.

The actual turnover time for automated clearing of the oldest histogramsat the end of each data collection time period would be programmable(e.g., midnight of the patient's time zone for a one day data collectiontime period). If the manual clearing function is used, it is envisionedthat the current section of histogram memory would still be used untilthe next turnover time.

FIG. 6A is an example of a histogram set 600 consisting of fivehistograms 601 through 605 inclusive representing an example of aprogrammer display screen of a single section of histogram data memory43 of FIG. 3 for a single data collection time period (viz., one day).In FIG. 6A, the horizontal scale is the ST deviation (i.e., ST segmentvoltage minus PQ segment voltage) as a percent of the R height,V.sub.PQR (519) of FIG. 4. Also in FIG. 6A, the vertical scale of eachhistogram 601 through 605 is the number of beats in the data collectiontime period (viz., one day) where the ST deviation was in one of theranges listed on the horizontal scale of the histogram. Each of the fivehistograms 601 through 605 represents all the beats processed (duringthe data collection time period of one day) that had R-R intervalscorresponding to the heart rate range for that histogram. It isenvisioned that the heart rate (or R-R interval) ranges for eachhistogram 601 through 605 may be either permanently set or programmableusing the physician's programmer 68 of FIG. 1. In the histograms 601through 605 each bin represents a range of ST deviation expressed as apercentage of the R height, V.sub.PQR (519) as shown in FIG. 4. Each binrepresents the shown value of −60, −55, −50, . . . +60, in percent of Rheight plus or minus 2.5%. Therefore, each bin covers a range (i.e., aclass interval) of 5% of the R height 519. The bin showing the value 5(i.e., +5%) in histogram 601 would be incremented by one every time abeat with an R-R interval corresponding to a heart rate of 50 to 80 bpmhad an ST deviation between 2.5% and <7.5% of the R height of that beat.The next higher bin would be 7.5% to <12.5% of the R height, and so on.It is also envisioned that instead of using the R height 519 of eachbeat as the reference, the average R height of a multiplicity of beatsof a baseline electrogram segment would be used as a reference.

Although the heart rate range for histogram 602 in FIG. 6A is shown as81 to 100 bpm, the cardiotracker will classify any beat whose R-Rinterval corresponds to a heart rate greater than 80 bpm and less thanor equal to 100 bpm as belonging in this heart rate range. Similarly theheart rate range labels of 101 to 120 bpm (histogram 603), 121 to 140bpm (histogram 604) and 141 to 160 bpm (histogram 605) will includebeats with R-R intervals corresponding to heart rates of >100 to <120bpm, >120 to <140 bpm and >140 to <160 bpm. This correspondence is alsoapplied to the charts in FIGS. 6B, 7A, 7B, 8A and 8B wherever heart rateranges are specified.

The technique of expressing ST deviation as a percentage of R height 519compensates for signal level variations from causes such as long termchanges in electrode impedance or changes in the gain of an amplifier.As an alternative, it is also envisioned that the actual voltage orsignal level or the percentage of a preset maximum signal level for theST deviation (e.g., millivolts) could be used as the range for each binin the histograms 601 through 605. For example, the bins in 601 mightrepresent between −60% to +60% of a maximum signal level of 10millivolts. Thus the bin labeled 5 would be incremented if the STdeviation was between 2.5% and 7.5% of 10 millivolts (i.e., 0.25 to 0.75millivolts). The technique described here will work with preset binranges. Preferably, this invention envisions bin ranges that can be setby the physician using the physician's programmer 68 of FIG. 1.

Also shown in FIG. 6A are the median (or average) values 611 through 615inclusive of the histograms 601 through 605 respectively. The medianvalue and number of beats counted in a histogram are useful extracteddata that would typically be saved in the extracted histogram datamemory 439 of FIG. 5. The medians and numbers of beats can also be usedto compute moving averages by either the cardiotracker 5 or programmer68 of FIG. 1. It is envisioned that comparison of the medians and/or themoving averages to pre-set thresholds can be used to alert the patientto a significant change in their cardiovascular condition.

FIG. 6B shows a set of histograms 650 consisting of the histograms 651,652 and 653 at three different ranges of heart rate (50 to 80, 81 to 100and 101 to 120 bpm) for the heart signal parameter QRS voltagecalculated as a percent deviation from the baseline QRS voltage. In FIG.6B, the horizontal scale represents 41 histogram bins (from −20% to+20%) with each bin corresponding to the labeled percent deviation ofQRS voltage from the baseline QRS voltage plus or minus 1%. Also in FIG.6B, the vertical scale represents the number of heart beats whosepercentage deviation from the baseline QRS voltage fell within each ofthe 41 bins during the data collection time period (e.g., one day). Forexample, for the histogram 651, in the bin labeled “−2” there were 3,000recorded beats that had a percentage difference between the measured QRSvoltage and the baseline QRS voltage between −3% and −1%. For example,if the baseline QRS voltage was 10 millivolts, histogram 651 shows thatthere were 3,000 beats with measured QRS voltage between 9.7 and 9.9millivolts. Similarly, the bin to the right of the −2% bin of histogram651 indicates that approximately 600 beats during the data collectiontime period had a QRS voltage within .+−0.1% of the baseline QRSvoltage.

The dashed lines 661, 662 and 663 represent the average values −2%, −4%and −8% of the histograms 651, 652 and 653 respectively. The averagevalue dashed lines 661, 662 and 663 represent respectively the median(or mean) values of the percent QRS voltage deviation for threedifferent heart rate ranges, namely: 50-80 bpm, >80-100 bpm and >100 to120 bpm for the histograms 651, 652 and 653. The heart rate ranges canbe set and adjusted by the medical practitioner using the programmer 68of FIG. 1.

FIG. 7A is a histogram display 700 that shows five different heat rateranges of histograms for three different days 701, 703 and 707. Thisrepresentation would typically be shown as a screen on the physician'sprogrammer 68 of FIG. 1. The display 700 of FIG. 7A would allow thephysician to examine trends in the ST deviation vs. heart rate overtime. This example clearly shows in day 7 (chart 707) that there is asignificant change in the distribution of ST deviation at higher heartrates as compared with days 1 and 3. This would be indicative of anarrowing or partial occlusion of one or more coronary arteries in theheart. Although this is a good way to look at changes between twodifferent time periods, the display of FIG. 8A is a preferred means toclearly see such changes. It is also envisioned that instead of thedistributions of ST deviation as shown in FIG. 7A, the average or medianST deviations for each heart rate range could be displayed as a singlevertical bar or line.

FIG. 7B is a histogram display 750 that shows three different heart rateranges for three different days 751, 753 and 757. Comparable to FIG. 7A,FIG. 7B shows the histograms for QRS voltage for a multiplicity of beatsplotted as a percent deviation from the baseline QRS voltage.

FIG. 8A is a graphical representation 800 of the five day moving averageof the average daily ST deviation for each of five heart rate ranges 801through 805 inclusive for a period of 26 weeks (6 months). The display800 as shown in FIG. 8A, would be of tremendous value to a cardiologistin recognizing a gradual but potentially life threatening change in apatient's cardiovascular condition. As a patient with the cardiotracker5 of FIG. 1 goes about daily activities their heart rate will go up anddown. Each beat analyzed by the cardiotracker (typically between 6 and80 beats in any particular minute) will increment the appropriate heartrate range related histogram allowing the cardiotracker 5 to store thedaily distributions of ST deviation in the five different heart rates.While the cardiotracker 5 may only store the histogram data for a weekor two, the extracted histogram data memory 439 of FIG. 5 could be usedto store extracted histogram data for a much longer period of time. Infact, the use of extracted histogram data is an extremely efficient wayto track the changes in heart signal parameters over an extended periodof time. For example storing the average ST deviation and number ofbeats in each of five daily histograms (5 heart rate ranges) requiresonly 15 bytes per day within the extracted histogram data memory 439.This translates to approximately 450 bytes per month and 5,500 bytes peryear. This efficient data storage can be compared with electrogram datastorage where at 200 samples per second, 30 seconds of electrogramstorage requires 6,000 bytes of data storage.

The display 800 could result from calculations made by the programmer 68of FIG. 1 after downloading six months worth of daily histograms orextracted histogram data from the cardiotracker 5. Alternatively, theprogrammer 68 could combine data downloaded from the cardiotracker 5 onmultiple occasions. Moving averages could also be calculated within thecardiotracker 5 or within the programmer 68 from the daily average ormedian value for ST deviation using the beat count extracted from thehistogram data. Such calculations would not overly tax the powerconsumption on the cardiotracker 5 as the calculations would require atmost a few seconds of processor time per day.

It is also envisioned that the cardiologist might set an alarm threshold820 for any or all heart rate range curves so that when one or more ofthe five day moving averages of ST deviation crosses the limit, thepatient would be alerted. Different thresholds for each heart rate rangecould also be implemented. In the example of FIG. 8A, the alarmthreshold 820 for the 121-140 bpm heart rate range 804 was set to −12%of the R height, and a SEE DOCTOR ALERT would have been initiated by thecardiotracker 5 two weeks before the current date. It is envisioned thatthe programmer 68 would allow the physician to set these detectionthresholds. The programmer 68 would also allow the physician to specifywhat type of alarm will be generated by the cardiotracker 5 if thedetection threshold is passed, e.g., either a SEE DOCTOR ALERT or anEMERGENCY ALARM. It is also envisioned that detection thresholds couldbe set for the slope of the curves of FIG. 8A so that significantdownward slope of ST deviation would initiate a patient alert. Also, itis envisioned that a combination of a specific value above the threshold820 when combined with a specific downward slope could also be used totrigger a SEE DOCTOR ALERT.

Instead of using the fixed threshold 820 for triggering a SEE DOCTORALERT from the 5 day moving average of the average ST deviation for eachheart rate range, an adaptive threshold that is based on the differencebetween the maximum and minimum of the 5 day moving average curvesexceeding a preset threshold is a preferred embodiment for the presentinvention.

The processing of extracted histogram data would typically be performedonce per day although longer and shorter data collection time periodsare also envisioned. An example of the extraction process for average STdeviation would be as follows:

1. Once per data collection time period (e.g., once per day), the STdeviation histogram data collected during the previous data collectiontime period is summarized, stored and analyzed. For each heart raterange, estimates are made of the average (e.g., mean and/or median) STdeviation, the average −1 sigma and the average +1 sigma of the STdeviation.

2. Other data, e.g., number of analyzed beats in each heart rate rangeand the average 24 hour baseline signal amplitude (e.g., R height or QRSvoltage) may also be stored as part of the summary data.

3. An N day moving average (N is typically between 1 and 30) of thedaily average (e.g., mean or median) ST deviation for each heart raterange is then determined, along with the maximum and minimum values ofthe N day moving averages for each heart rate range.

If the difference between the maximum moving average and the minimummoving average of the ST deviation for any of the ST deviation movingaverage curves 801 through 805 exceeds a preset threshold, an STdeviation histogram trending event for that heart rate range can bedetected. If enabled, a SEE DOCTOR ALERT would then be triggered.

The hour at which the daily extraction would occur is programmable bythe doctor so that detection of such a trending event would trigger theSEE DOCTOR ALERT at time that is convenient to the patient (e.g., notwhile he would be sleeping). Once a SEE DOCTOR ALERT has been triggeredand the patient has had therapy (e.g., a stent or angioplasty procedure)that relieves the ST depression (or elevation) the programmer 68 of FIG.1 can be used to reset the start date for future histogram trendinganalysis so that the ST shift data that caused the alert in the past isnot used in future analysis. An alternative technique to accomplish thisis to clear all previously stored histogram data from the cardiotrackermemory once the ST shift has been treated. Therefore any new analysiswould not include the data that caused the histogram trending event. Theprior data would however, remain in the programmer 68 for later reviewand tracking of the patient's history.

For example, once per day at noon, to avoid alerting the patient when hemight be asleep, the cardiotracker could calculate the daily average(mean or median) ST deviation from the histogram for each heart raterange (e.g., 601 through 605) of FIG. 6A. The cardiotracker would thencalculate the 5 day moving average that includes the just calculateddaily average ST deviation and the averages from the four previous days.The cardiotracker could then identify the maximum and minimum values ofthe moving average data for each heart rate range after a start date setby the programmer 68. If the difference between the maximum and minimumvalues exceeds a preset threshold for any heart rate range, then ahistogram trending event is detected and, if enabled, a SEE DOCTOR ALERTwould be triggered in the implanted cardiotracker 5.

FIG. 8B illustrates a display 850 on the physician's programmer 68 forthe median (or mean) value of the percent deviation of QRS voltage overa six month period compared to a baseline QRS voltage. The display 850shows the percent deviation for QRS voltage for three different heartrate ranges corresponding to the heart rate ranges shown for FIGS. 6Band 7B. The three curves, 851, 852 and 853 correspond respectively tothe heart rate ranges of 50-80 bpm, 81-100 bpm and 101 to 120 bpm.

It is expected that the display 850 of FIG. 8B would be of great valueto doctors who treat heart transplant patients. Specifically, it hasbeen shown by Warnecke, et al that a decrease of 8% in the QRS voltagefrom a baseline QRS voltage value from a time when the heart is notbeing rejected can indicate rejection of a transplanted heart at anearly enough time to change the patient's medication to save that heart.The present “gold standard” for detecting rejection is a biopsy that(starting two years after implant) is typically carried out only onceeach six month time period. This biopsy is done in a catheterizationlaboratory and it is typically difficult for the patient and quiteexpensive. Also, if rejection occurs starting at some time between thesix month biopsy procedures, then that early detection of rejection willnot be possible. If however, a patient has an implanted cardiotracker 5that has an alarm that is triggered by the −8% decrease in QRS voltage,then that SEE DOCTOR ALERT setting 860 as shown in FIG. 8B will occurand the heart in that transplant patient can be saved by appropriatemedication therapies. It is envisioned that the setting of the level 860for triggering a SEE DOCTOR ALERT could be between −1% and −20% belowthe baseline value of the QRS voltage. Furthermore, one could combine anegative slope of any of the curves of FIG. 8B with a higher value fortriggering the SEE DOCTOR ALERT. For example, if a slow descent of thepercent deviation of QRS voltage utilized a −8% drop as the level to setoff the SEE DOCTOR ALERT, it is envisioned that a level of (let us say)−6% could be used to set off the SEE DOCTOR ALERT if the downward slopecorresponded to (let us say) a −1% per week decrease in QRS voltage.Thus the patient would be warned two weeks earlier that he is going toreach the level of −8% when his doctor would prescribe a change in thepatient's medication regime.

While it may be sufficient to detect transplant rejection when thedeviation of average daily QRS voltage as compared to the baseline QRSvoltage exceeds a preset threshold for a single day, it may be morereliable to require that the threshold be exceeded for two or moreconsecutive days. An example of the extraction process for average (meanor median) QRS voltage would be as follows:

1. Once per data collection time period (e.g., once per day), the QRSvoltage data collected during the previous data collection time periodis summarized, stored and analyzed. For each heart rate range,calculations are made of the average (e.g., mean and/or median) QRSvoltage and the average −1 sigma and average +1 sigma deviations of theQRS voltage.

2. Other data, e.g., number of analyzed beats in each heart rate rangebaseline R height for the past 24 hours could also be stored as part ofthe summary data.

If the average QRS voltage has declined more than a preset percentage ofthe baseline QRS voltage, a transplant rejection event for that heartrate range will be detected. If enabled, a SEE DOCTOR ALERT would thenbe triggered. The baseline QRS voltage is an average QRS voltagecaptured at an earlier time when the transplanted heart was notexperiencing rejection. It is also envisioned that to reduce thepossibility of a false positive detection, a SEE DOCTOR ALERT would onlybe triggered after a specified number of successive transplant rejectionevents. For example, it might require two or three successive transplantrejection events to trigger the alert.

The hour at which the daily extraction of collected data would occur isprogrammable by the doctor so that detection of such an event wouldtrigger the SEE DOCTOR ALERT at a time that is convenient to the patient(e.g., not while the patient would be sleeping). Once a SEE DOCTOR ALERThas been triggered and the patient has had therapy (e.g., an increase incyclosporine) that reverses the rejection episode, the programmer 68 ofFIG. 1 can be used to reset the baseline QRS voltage so that the datathat caused the alert is in the past and is not used in future analysis.

For example, once per day at noon, the cardiotracker will calculate thedaily average (mean or median) QRS voltage from the histogram for eachheart rate range (e.g., the heart rate ranges 651 through 653 of FIG.6B). If the difference between the recently calculated average QRSvoltage and the baseline QRS voltage exceeds a preset threshold 860 forany heart rate range, then a transplant rejection event is detected andif enabled, a SEE DOCTOR ALERT (or possibly an EMERGENCY ALARM) istriggered.

Although a decline in the average QRS voltage is cited here as a knownmeans for early detection of rejection for a transplanted heart, it isalso envisioned that some other heart signal parameter may be equally orbetter suited for that purpose. Specifically, ST deviation or ST shift,R wave slope, QRS complex width or another heart signal parameter couldbe used for the early detection of rejection of a transplanted heart.Furthermore, it is envisioned to place an accelerometer onto the end ofan epicardial or endocardial lead, which end is firmly attached to theheart muscle, to detect a change in heart wall motion that could beindicative of early rejection. The combination of a means to measureheart wall motion with a second means to detect a change in a heartsignal parameter is also envisioned as a means for early detection ofthe rejection of a transplanted heart.

Although ST deviation and QRS voltage have been the primary examplesused here for histogram data collected based on a patient's heart rate,it is envisioned that any other heart signal parameter measured orcalculated can be usefully used with this histogram methodology.Examples of such parameters include QRS or RS complex width, ST shift(ST deviation compared to a baseline ST deviation), R wave width, T waveshape, T wave alternans, changes in R-R interval variability and numberof overly long R-R intervals. These parameters may be monitoredindependent of the patient's heart rate, or separate histograms could beused for each of multiple heart rate ranges.

Although the present invention has described the use of histogram memoryfor cardiovascular electrical signals, these techniques are alsoapplicable for electrical signals collected using electrodes from otherportions of the human body. Such electrical signals include signals fromthe human brain, gastrointestinal tract, the liver, the pancreas andmusculature. Any of these organs may (for example) have a change intheir electrical signal that might indicate an early stage of rejection.Furthermore, although only electrogram related histograms have beendescribed herein, it should be understood that other measurementsincluding measurements by heart motion sensors, temperatures at certainplaces in the body and devices to measure pressure and/or pO.sub.2 maybe used to generate histograms of cardiovascular condition of thepatient.

It is also envisioned that all of the processing techniques describedherein for an implantable cardiotracker are applicable to a trackersystem configuration using skin surface electrodes and a non-implantedcardiotracker. For systems that were totally external to the patient,the term “electrogram” would be replaced by the term“electrocardiogram”. Thus the cardiotracker device described in FIGS. 1through 3 inclusive would also function as a monitoring device that iscompletely external to the patient.

It is important to note that many of the functions of the tracker systemas described herein that are programmable by a medical practitionercould be preset in manufacture to typical settings that are useful formost patients. Thus the doctor could use this default mode instead oftrying to set particular alarm parameters for a particular patient.Furthermore, the physician's programmer 68 could have a default mode torestore all the settings of either or both the cardiotracker 5 andexternal alarm system 60 to values that are recommended by themanufacturer. There may also be separate default settings for men andwoman and others that would be related to a specific medical problemthat the patient has.

Although the histogram technique is a preferred embodiment of thepresent invention as it greatly reduces the amount of memory needed tostore the values of a heart signal parameter for each beat analyzedduring a data collection time period, it is also envisioned that theeach measured or calculated value of one or more heart signal parameterscould be directly stored in memory. For example, the value of STdeviation would be measured for each beat during a one hour datacollection time period (e.g., during a stress test). These values wouldall be stored in memory and at the end of the data collection timeperiod, the average ST deviation for each heart rate range could becalculated from the stored values. This technique would be of greatestvalue where the data collection time period is shorter than a day.

Various other modifications, adaptations, and alternative designs are ofcourse possible in light of the above teachings. Therefore, it should beunderstood at this time that, within the scope of the appended claims,the invention can be practiced otherwise than as specifically describedherein.

1. A system for tracking the cardiovascular condition of a human patientthe system including: at least two electrodes positioned to sense theelectrical signal from the patient's heart, the electrical signalconsisting of a multiplicity of beats; a histogram memory includingmemory capacity to store at least one histogram, the at least onehistogram including at least two bins, each bin being a counter; and aprocessor to compute the amplitude of the ST segment of the electricalsignal for at least a portion of the multiplicity of beats, each beat ofthe portion of the multiplicity of beats that is processed being aprocessed beat, the processor also being designed to compute the R-Rinterval for each processed beat, the processor further being designedto increment one of the at least two bins of the at least one histogramwhere the choice of bin to be incremented is dependent on the computedamplitude of the ST segment for one or more processed beats, a histogramonly being incremented when a computed R-R interval associated with theone or more of the processed beats is within a particular heart raterange, any histogram stored in the histogram memory being incremented bythe processor only for a data collection time period.
 2. The system ofclaim 1 wherein the current data collection time period is 24 hours. 3.The system of claim 1 wherein the histogram memory is divided into aplurality of sections, and wherein one of the plurality of sectionsstores a current histogram, and wherein another of the plurality ofsections stores a past histogram associated with a prior data collectiontime period.
 4. The system of claim 3 wherein the processor isconfigured to obtain a new histogram during a new data collection timeperiod by overwriting the oldest of the plurality of sections.
 5. Thesystem of claim 1 wherein the ST segment amplitude is computed as thedifference between ST segment potential and PQ segment potential.
 6. Thesystem of claim 1 wherein the ST segment amplitude is normalized to Rwave amplitude.
 7. The system of claim 1 wherein the particular heartrate range is a range of normal heart rates.
 8. The system of claim 1wherein the particular heart rate range is a range of elevated heartrates.
 9. The system of claim 1 wherein the processor updates a currenthistogram separately for each of the processed beats.
 10. The system ofclaim 9 wherein the R-R interval is computed based on the period betweena beat and a preceding beat.
 11. The system of claim 10 wherein the R-Rinterval is computed by calculating the period between QRS fiducialpoints between successive beats.
 12. The system of claim 1 wherein theelectrodes are configured to be implanted within a body.
 13. The systemof claim 12 wherein at least one of the implanted electrodes is adaptedto be disposed in the heart.
 14. The system of claim 13 wherein the atleast one of the implanted electrodes is adapted to be disposed in theRV apex.
 15. The system of claim 1 wherein at least one of theelectrodes is attached to the patient's skin.
 16. The system of claim 1where the RR interval is calculated based on the heart rate measured forone or more processed beats.
 17. The system of claim 1 furthercomprising a housing adapted for implantation into a human body, andwherein the processor is disposed within the housing.
 18. The system ofclaim 1 wherein the processor is configured to periodically sample theelectrical signal, thereby defining electrical signal segments.
 19. Thesystem of claim 1 wherein each of the at least two bins has the samewidth.